This invention relates to a process for preparing organic pigments having excellent pigmentary properties and dispersibilities, especially when used to color polymeric materials.
Crude organic pigments are generally unsuitable for use as initially isolated and must be subjected to one or more additional finishing steps that modify particle size, particle shape, and/or crystal structure in such a way that provides good pigmentary quality. See, for example, K. Merkle and H. Schafer, "Surface Treatment of Organic Pigments" in Pigment Handbook, Vol. III (New York: John Wiley & Sons, Inc., 1973), page 157; R. B. McKay, "The Development of Organic Pigments with Particular Reference to Physical Form and Consequent Behavior in Use" in Rev. Prog. Coloration, 10, 25-32 (1979); and R. B. McKay, "Control of the application performance of classical organic pigments" in JOCCA, 89-93 (1989).
The most commonly used conditioning methods involve dissolving or suspending the crude pigment in suitable liquids, especially strong mineral acids, and/or milling. E.g., W. Carr, "Improving the Physical Properties of Pigments" in Pigment Handbook, Vol. III (New York: John Wiley & Sons, 1973), pages 29-31; see also U.S. Pat. Nos. 4,493,734, 4,404,036, 4,298,399, 4,287,000, 4,077,972, 3,936,315, and 3,264,300, European Patent Applications 638,615 and 69,895, and British Patent 1,438,921. Conditioning with a strong acid involves treating the crude pigment with aqueous mineral acid (preferably sulfuric acid) in a process known as "acid pasting" (in which an acidic solution containing protonated pigment is formed) or "acid swelling" (in which a suspension of protonated pigment is formed). It is also possible to condition the crude pigment using a liquid other than a strong acid, such as organic acids, alcohols (including glycols), ethers, ketones, aromatics, esters, and amides. Milling can be carried out using dry-milling methods and wet-milling methods in the presence of water and/or various organic liquids, with or without additives. Crude organic pigments can also be premilled and then treated with an organic liquid. In salt grinding, a crude organic pigment is ground in the presence of an inorganic salt that is subsequently removed by washing. Each of these methods, however, is attended by practical difficulties and does not always produce pigments having desirable combinations of color properties and dispersibility.
U.S. Pat. No. 4,970,248 discloses a process for converting the alpha form of a particular isoindolineazine pigment into the gamma form by salt grinding or kneading the pigment in the presence of solvents and/or surfactants and optional texture improving agents. The optional texture improving agents are described as being fatty acids or derivatives having at least twelve carbon atoms, quaternary ammonium salts, plasticizers, waxes, resin acids, or C.sub.12 -C.sub.18 paraffin disulfonic acids, alkylphenols, or alcohols that can be added before, during, or after salt kneading or grinding. Lower carboxylic acids are not disclosed. In addition, the patent is directed only to a method for changing one crystal form to another crystal form.
U.S. Pat. No. 5,776,238 discloses a process for converting a crude pigment into a pigmentary form by kneading the crude pigment in the presence of a liquid carboxylic acid and an optional salt. The liquid carboxylic acid is used instead of other types of organic solvent.
It has now been found that conditioning an organic pigment in a stepwise salt grinding process using a non-solvent liquid followed by a C.sub.8 -C.sub.10 alkanoic acid produces excellent dispersibility without further conditioning steps.